1. Field of the Invention
The field of the invention generally pertains to systems and methods for controlling energy distribution at local sites.
2. Background
Electrical utilities face particular challenges in meeting continuously changing customer load demands. At least two related reasons exist for these challenges. First, power demands can fluctuate substantially from day to day or hour to hour, making it difficult for utilities to ensure that they have enough capacity to meet demand. These fluctuations in energy demand may arise from ordinary cyclic energy usage patterns (for example peaking in the afternoon), or else can result from an unexpected change in the balance between energy supply and demand, such as where, for example, a power generator linked to the power grid unexpectedly goes down, large energy users go on or off line, or a fault occurs somewhere in the distribution system.
A second factor contributing to the challenges faced by power utilities is the fact that power consumption in local areas tends to grow over time, gradually placing increasing burdens on electrical utilities to meet the growing demand. Because the construction of new power plants is very costly and must comply with a variety of governmental regulations, it is possible for a local or even large geographic region to find itself without the power capacity to supply its current or anticipated future demand.
A major challenge for utility companies is handling peak energy demands. This is because the energy supplied by power utilities must be sufficient to meet the energy demand moment by moment, and peak demands place the greatest strain on the power distribution system. When energy demand outstrips available supply, disruptive events such as power blackouts, brownouts or interruptions can occur. Not only can such events cause substantial inconvenience to large numbers of people and businesses, but they can also be dangerous or life-threatening—where, for example, the power supply for hospitals or critical home care medical equipment is compromised.
Historically, when power utilities serving a locality have been faced with a severe energy situation caused by high demand, their options have been extremely limited. Power utilities can, for example, request that consumers conserve energy, but not all consumers follow such requests and, in any event, conservation has not tended to provide a complete solution for energy supply problems. Power utilities can attempt to satisfy peak demands by purchasing available energy from a third party source connected to the power grid, but such purchases, particularly at peak demand times, can be extremely costly as energy suppliers often demand a premium when demand is high. Another option is for power utilities to build additional power plants, but building power plants takes substantial time and investment, and may require approvals from state and/or federal government authorities as well as consumer associations.
To help reduce peak power demand and thus ward off costs associated with new power plants or premium energy purchases, various attempts have been made to develop load management systems which control peak demand on the power generating equipment by temporarily turning off certain customer loads when deemed necessary to avoid a blackout or similar power interruption. Generally, the types of customer loads that are regulated in this manner involve non-critical electrical equipment such as air conditions, electric heaters, and the like.
One type of load management system, for example, uses ripple tone injection to send coded pulses over the utility's power lines. The coded pulses may be applied to the utility power lines by way of an electromechanical ripple control transmitter, which may consist of a motor/alternator operating through thyristor static switches, or by way of a step-up transformer selectively connected to the utility power lines through a passband circuit tuned to the frequency of the coded pulse signal. At the customer sites, receivers interpret the coded pulses and perform desired command functions—e.g., turning off the customer load(s).
An example of a particular system for load management is described in U.S. Pat. No. 4,264,960. As set forth in that patent, a plurality of substation injection units, under control of a master control station, transmit pulse coded signals on the utility power lines. Remote receiver units positioned at customer loads control the on and off states of the loads in response to the signals received over the utility power lines from the substation injection units, by activating latchable single-pole contacts. Different types of loads are organized into load control groups (e.g., electrical hot water heaters, air conditioner compressors, street lights, etc.). The master control station independently controls the various different types of loads through different pulse control signals. Each remote receiver unit is precoded so that it responds to one and only one pulse code signal. In order to control different types of loads (e.g., hot water heater and air conditioning compressor) at the same location, separately encoded remote receiver units at the location are required. The master control station turns load groups on and off in order to implement a load management strategy, as determined by a system operator.
A variety of drawbacks or limitations exist with conventional techniques for load management in large-scale power distribution systems. A major drawback is that shut-off commands from the power utility to the remote customer sites are generally propagated over the same lines that carry high-voltage electricity. Because transformers are used to relay electrical signals across power lines, it can be difficult to pass data (e.g., shut-off commands or other control signals) over power lines. Moreover, noise or interference can prevent proper reception of shut-off commands or other control signals. Any inductance at the customer load can generate large harmonics, which can easily match the control signal frequency, thus blocking out control signals or possibly causing “false alarms.” A simple household device such as an electric oven can disrupt the reception of control signals over power lines. Over a large area, since all loads inject noise into the power distribution system, the cumulative interference or noise effect can be substantial. Thus, using power lines to distribute control signals can be quite problematic, because of the many sources of noise and interference. Sophisticated digital signal processing techniques might be used to filter out the noise or interference and reconstruct control signals, but such techniques are complicated and would generally require that a receiver be quite costly.
Another drawback with conventional techniques for load management is the lack of control either at the utility or consumer level. In situations where the utility is forced to shut off power to one or more regions (e.g., by causing a rolling blackout) in order to prevent peak demand from causing a catastrophic blackout or damaging power generation or distribution equipment, power customers typically have little or no control over which loads get shed. Rather, a complete shut-down of the customer's power usually occurs for those customers within a region subject to a rolling blackout. Even in those situations where the utility has pre-configured the customer's wiring so that certain isolated loads (usually an air conditioner or electric water heater) can be dynamically shed at peak power times, neither the utility nor the customer can easily alter which loads get shed unless the customer's wiring is re-configured. Where the customer loads are collectively grouped into different load control groups, the utility may be able to shed certain types of loads (e.g., all air conditioners) en masse, but the choice is generally made by the utility based upon its overall power demands and management strategy, with little or no control being available to the customer (other than perhaps initially giving permission to the utility to shut down a specific load, such as an air conditioning unit, before the utility pre-configures the wiring to control the specific load as part of a larger group of similar loads).
Another problem that remains insufficiently addressed by conventional load management techniques is the fact that power interruptions, brownouts or blackouts generally occur with little or no warning to power customer. In some cases, where unusually large demand can be forecasted, electrical utilities have been able to provide warnings to power customers that a blackout or power interruption is likely within a certain upcoming period of time—e.g., within the next several hour period, or next 24 or 48 hour period. However, power interruption or blackout warnings are typically so broad and vague in nature as to be of limited or no value to power customers, who are left with uncertainty as to whether or not their power will go out and if so, exactly when. Moreover, since power interruption or blackout warnings are normally broadcast by radio or television, customers who are not tuned in by radio or television to the broadcast stations can easily miss the warnings and not realize that a power interruption or blackout is imminent.
Certain power management techniques have been proposed for controlling power consumption at a specific local site (e.g., a factory), but such systems are usually isolated and operate independently of the power utility. An example of one power management system is described, for example, in U.S. Pat. No. 4,216,384. According to a representative technique described therein, the various main power lines of the installation or site are monitored for energy usage, and a control circuit selectively disconnects loads when the total energy being drawn at the installation or site exceeds a specified maximum. While ostensibly having the effect of reducing overall power consumption at the installation or site, a drawback of these types of power management systems is that they can be relatively complex and costly. For example, the power management system described in U.S. Pat. No. 4,216,384 utilizes a set of transformers to independently monitor various main power lines, a bank of LED-triggered Triacs to selectively engage various customer loads, programmable control circuitry, automatic priority realignment circuitry, and so on. Because of their relative cost and complexity, these types of local power management systems are not very suitable for widespread use, particularly for ordinary residential use or other cost-sensitive applications. Moreover, their operation is very localized in effect, and cannot be controlled from a central location such as the power utility itself.
In addition to the foregoing limitations and drawbacks, conventional power and load management strategies are limited by the available circuits and switches which are used in some applications to control actual power delivery at local sites. One common type of power switch, for example, for connecting and disconnecting power sources to loads is a circuit breaker, which functions to prevent an excessive amount of current from being drawn from the power source or into the load by breaking the electrical circuit path between the source and load when the current limit is reached. A typical circuit breaker has a bimetal arm through which travels a power signal from the source to the load. One end of the bimetal arm is connected to the power signal line, while the other end of the bimetal arm is connected to an electrical conductor from which the power can be distributed to the load. When too much current travels through the bimetal arm, the heat from the current causes the bimetal arm to deform or bend in a predictable manner, which causes the bimetal arm to break contact with the electrical conductor, resulting in a break between the power signal and the load. In this manner, the source and load are both protected from currents which exceed a certain limit.
While circuit breakers are useful for protecting against high current levels, they are generally passive circuit elements whose response depends entirely upon the amount of power being drawn by the load. They typically do not provide active control of a power signal line. However, some resettable circuit breakers have been proposed, which utilize, for example, a spring-operated mechanism allowing a remote operator to open and close the contacts of the circuit breaker. An example of such a circuit breaker is disclosed in U.S. Pat. No. 3,883,781 issued to J. Cotton.
Other types of remotely controlled or operated circuit breakers are described, for example, in U.S. Pat. No. 5,381,121 to Peter et al., and U.S. Pat. No. 4,625,190 to Wafer et al. These circuit breakers involve rather elaborate mechanisms that, due to their complexity, would be expensive to manufacture and potentially subject to mechanical wear or failure.
Besides circuit breakers, other types of circuits have been utilized in controlling power signals. However, these other types of circuits have drawbacks as well. For example, solid state switches (e.g., transistors or silicon-controlled rectifiers (SCRs)) can be used as switches between a power source and load, for controlling distribution of the power signal to the load. However, transistors and SCRs generally have limited power ratings and, at high current levels, can become damaged or shorted. Moreover, transistors or SCRs with high power ratings can be relatively expensive.
It would therefore be advantageous to provide a load management system that overcomes one or more of the foregoing problems, limitations or disadvantages. It would further be advantageous to provide a load management system that gives more flexibility to power utilities and/or consumers, that is not subject to the noise and interference effects caused by transmitting data over power lines, and does not require a relatively expensive receiver. It would also be advantageous to provide a load management system that uses a controllable electronic switch capable of selectively connecting or disconnecting a power source to a load and, in particular, a switch that is reliable, durable, and low-cost, and that can handle relatively high power demands, such as may be required for residential or commercial applications.